The objective of the proposed research are to elucidate the mechanism of action of cyclic GMP on activating cyclic GMP-dependent protein kinase (G-PK), presumably the physiologic target enzyme for cyclic GMP, from the heart and its functional roles in biology and pathology of the heart. The enzyme will be extensively purified from the fetal calf heart (about 5 times richer in G-PK than the adult heart) and its properties studied in detail. It is hoped to establish that cyclic GMP acts to cause dissociation of the holoenzyme to respective catalytic and regulatory (cyclic GMP-binding) subunits and that the activator component of protein kinase modulator is essential for the activity of the holoenzyme and the catalytic subunit. Attempts will be made to find the sites of action of G-PK, and to correlate phosphorylation of the potential "natural substrates" (such as plasma membrane, sarcoplasmic reticulum, components of actomyosin, and phosphoprotein phosphatase, etc.) with subsequent modifications in their properties. Attempts will be also made to establish patterns of changes in the G-PK level and cyclic GMP content, compared to those of cyclic AMP-dependent protein kinase (A-PK) and cyclic AMP, in the heart under certain physiological and pathological conditions. For the physiologic aspect of the studies, the developing heart (fetus, neonate and adult) of guinea pigs and the "super" heart from the treadmill-exercised rats will be used. The pathological role of G-PK will be explored by examining the alterations in the G-PK level and cyclic GMP content in experimental hypertrophic and failing heart (such as in spontaneously hypertensive rats, rats treated with DOCA-salt, isoproterenol or thyroxine, or subjected to aortic ligation), in myocardial hyperplasia (young anemic rats) and in cardiomyopathy (BIO 14.6 Syrian hamsters). The data obtained would also shed light on the cause-effect relationship between the abberrations in the cyclic nucleotide systems and genesis of cardiac disorders.